**5.5 Building the digester**

In case of commercial biogas plants, the digester is huge as its diameter may reach 25 m; therefore, the concrete structure should be reinforced (Fig. 11). Hence, the iron rods are used to build 2 iron grids to reinforce the digester wall starting from the digester bottom plate. The standard length of iron rods is 12 m. The standard iron rods are cut to shorter iron rods, and they are then used to build up the tank. Subsequently, either wood panels or preconstructed metal sheets are used to enclose the iron grids and to form a container for the fluid concrete. When the digester wall is built, about one third of the internal wall of the tank is covered by a protection layer in order to protect the internal face of the wall against corrosion.

Fig. 9. Pouring the concrete mixture for a commercial biogas plant (BIOGAS NORD GmbH)

In case of commercial biogas plants, the digester is huge as its diameter may reach 25 m; therefore, the concrete structure should be reinforced (Fig. 11). Hence, the iron rods are used to build 2 iron grids to reinforce the digester wall starting from the digester bottom plate. The standard length of iron rods is 12 m. The standard iron rods are cut to shorter iron rods, and they are then used to build up the tank. Subsequently, either wood panels or preconstructed metal sheets are used to enclose the iron grids and to form a container for the fluid concrete. When the digester wall is built, about one third of the internal wall of the tank is covered by a protection layer in order to protect the internal face of the wall against

Fig. 10. Concrete bottom of a household unit

**5.5 Building the digester** 

corrosion.

Fig. 11. Building the digester wall for a commercial biogas plant

In case of household units, burnt-clay bricks are used to build the digester (Fig. 12) and they should be able to tolerate a pressure up to 100 kg cm-2 owing to the fact that the walls of the digester are exposed to the pressures of the soil and the moving equipment near to the digester. A mortar of cement and sand mixture by 1:4 is used. The construction works are followed up till the appropriate height, and the entry or exit holes of the pipes are blocked by a filling material.

Fig. 12. Building the digester wall of a household unit using burnt-clay bricks
